Optical sheet

ABSTRACT

There is provided an optical sheet for use as a display device surface, which has a functional layer on at least one side of a base material and has a diffusion factor on the outer surface and/or interior of the functional layer, wherein the relationship represented by the following formula (I) is satisfied.
 
0.16&lt; S×β &lt;0.70  (I)
     S: Diffuse regular reflection intensity/regular reflection intensity of base material   β: The diffusion angle exhibiting ⅓ of the diffusion intensity, obtained by extrapolating a straight line connecting the reflection intensity for diffuse regular reflection intensity at ±2 degrees and the reflection intensity for diffuse regular reflection at ±1 degree, to the diffuse regular reflection angle.

TECHNICAL FIELD

The present invention relates to an optical sheet with excellentcontrast.

BACKGROUND ART

Optical sheets used for display device surfaces have layers withfunctions such as an anti-glare property, antistatic property andantifouling property laminated as functional layers on the observer sideof a base material. To exhibit these functions, for example, in order toimpart an anti-glare property, methods of forming an irregular shape inthe surface layer or adding diffusion particles to the resin forming thesurface layer are employed. Conductive fine particles or a conductiveresin may be added to impart an antistatic property, or afluorine-containing polymer or stain-proofing agent may be added inorder to impart an antifouling property. Since such diffusion particles,conductive fine particles and additives are not completely phase-solublewith surface layer-forming resins, an optical sheet that employs themhas a function of diffusing visible light. The irregular sections of thesurface layer also have the function of diffusing visible light.

In addition, irregularities larger than the visible light wavelength areformed in the surface layer, the base material back side and betweeneach layer in order to prevent interference patterns between opticalsheets and interference patterns between optical sheets and displaydevices, and such irregularities also have the function of diffusingvisible light.

According to the invention, such causes of visible light diffusion aredefined as “diffusion factors”, and the presence of such diffusionfactors causes the optical sheet to have reduced contrast due toreflection of external light. In other words, an optical sheet shouldmaintain the function of the optical sheet while preventing loss ofcontrast.

The haze value, or the ratio of the interior haze and total haze, iscommonly used as a simple method for evaluating contrast. Specifically,it has been considered that an optical sheet with low contrast reductioncan be produced by specifying the materials and controlling theproduction conditions in the optical sheet production process for alower haze value (see Japanese Unexamined Patent Application PublicationNo. 2002-267818, Japanese Unexamined Patent Application Publication No.2007-334294 and Japanese Unexamined Patent Application Publication No.2007-17626).

CITATION LIST Patent Literature SUMMARY OF THE INVENTION

However, contrast often differs even with the same haze value, and ithas been found that, with production using the haze value and the ratioof the interior haze and total haze as indexes, for example, it is notalways possible to stably produce a satisfactory optical sheet.

In light of these circumstances, it is an object of the presentinvention to provide an optical sheet with satisfactory contrast.

Contrast has hitherto been considered to depend on surface form, whichincludes the Ra, Rz, Sm and θa values for surface irregularities, or hasbeen considered to depend on the state of reflection of external lightthat is based on the difference in refractive index between the interiordispersing agent and binder resin, or interior diffusion particles. Inother words, the effect of interaction between surface irregularitiesand internal diffusion factors has not been considered.

The present inventors have found that the diffusion properties for lightpassing through and light reflected by diffusion particles from amongprojected light and external light incident to the diffusion particles,differs significantly depending on the difference in refractive indexbetween the interior diffusion particles and binder resin, as indicatedby 1 to 4 in FIG. 10, and that a larger difference in refractive indexbetween the diffusion particles and binder increases the reflected lightquantity by the diffusion particles and increases the diffusion angle,thus increasing the stray light quantity from projected light and thereflected light quantity from external light and lowering the contrast,and have further found that for projected light as well, thetransmission and reflectance properties for projected light passingthrough the diffusion particles, and the state of stray light generationwhich impairs resolution and contrast, differ significantly depending onthe positional relationship between the diffusion particles and surfaceirregularities, as indicated by 1-1 to 1-5 in FIG. 9-1, and that forexternal light as well, the state of stray light generation whichimpairs the reflectance properties and contrast for reflected light bythe diffusion particles from external light incident to the diffusionlayer interior, differ significantly depending on the positionalrelationship between the diffusion particles and surface irregularities,as indicated by 2-1 to 2-4 in FIG. 9-2, so that by combining the surfaceirregularity shape, the diffusion particle properties and therelationship between surface irregularities and interior diffusionparticles according to the optical sheet of the present application, itis possible to obtain an optical sheet with excellent contrast.

Also, when the positional relationship between the surfaceirregularities and diffusion particles is such that the diffusion ofexternal light reflected by the diffusion particles is large, as shownby the diffusion particles 2-2 in FIG. 9-2, the diffusion of projectedlight is also large tending to produce stray light, as indicated by 1-2in FIG. 9-1, thus also tending to result in contrast reduction due tothe projected light. That is, the magnitude relationship for contrastreduction by stray light from projected light can be considered toapproximate the reflectance properties for external light.

Contrast has hitherto been considered to depend on the haze value. Thehaze value, according to JIS K7136:2000 and ISO 14782:1999, is definedas “the percentage of transmitted light of at least 0.044 rad (2.5°)from an incident beam by forward scattering, of the transmitted lightpassing through a test piece”. That is, the haze value represents thepercentage of scattered light being scattered at least ±2.5° from anincident light beam, and it completely ignores the luminancedistribution by diffusion. In terms of extreme examples, optical sheetswith total mirror reflection (absolutely no diffusion) and opticalsheets with no mirror reflection but with all of the transmitted lightdiffused within ±2.5°, have haze values of 0.

On the other hand, for example, an optical sheet wherein the lightquantity within ±2.5° of the transmitted light is 30% and thetransmitted light scatters at an angle of 70-80°, and an optical sheetwherein the light quantity within ±2.5° of the transmitted light issimilarly 30% and the transmitted light scatters at an angle of 5-10°,both have haze values of 70%.

FIG. 1 shows the relationship between haze value and contrast, and FIG.2 shows the relationship between interior haze and total haze ratio andcontrast, as results verified for optical sheets fabricated underdifferent conditions. As shown in FIG. 1 and FIG. 2, optical sheets mayhave completely different contrast as evaluated by an observer, even ifthey have the same haze value and ratio between interior haze and totalhaze. The contrast referred to here is light-room contrast.

As a result of much diligent research based on this knowledge, thepresent inventors have found that it is possible to convenientlyevaluate contrast that has not been evaluable with the conventional hazevalue, by incorporating the concept of intensity distribution due todiffusion. More specifically, it was found that contrast can beconveniently evaluated by using as the index the product of the value Sas the diffuse regular reflection intensity of an optical sheet dividedby the regular reflection intensity of a base material with no diffusionfactor composing the optical sheet, and the diffusion angle β thatexhibits ⅓ of the diffusion intensity obtained by extrapolation of astraight line connecting the reflection intensity for diffuse regularreflection intensity at ±2° and the reflection intensity for diffuseregular reflection at ±1°, to the diffuse regular reflection angle, andthat an optical sheet with minimal reduction in contrast can beefficiently and stably produced by using the above as an index tocontrol the material selection and production conditions in the processof producing the optical sheet.

The present invention has been completed based on the knowledgedescribed above, and it encompasses the following modes.

-   (1) An optical sheet for use as a display device surface, which has    a functional layer on at least one side of a base material and has a    diffusion factor on the outer surface and/or interior of the    functional layer, wherein the relationship represented by the    following formula (I) is satisfied.    0.16<S×β<0.70  (I)-   S: Diffuse regular reflection intensity/regular reflection intensity    of base material-   β: The diffusion angle exhibiting ⅓ of the diffusion intensity,    obtained by extrapolating a straight line connecting the reflection    intensity for diffuse regular reflection intensity at ±2° and the    reflection intensity for diffuse regular reflection at ±1°, to the    diffuse regular reflection angle.-   (2) An optical sheet according to (1) above, wherein the following    formula (II) is satisfied.    0.26<S×β<0.70  (II)-   (3) An optical sheet according to (1) above, wherein the following    formula (III) is satisfied.    0.46<S×β<0.70  (III)-   (4) An optical sheet according to any one of (1) to (3) above,    wherein the display device is a liquid crystal display unit.-   (5) An optical sheet according to any one of (1) to (4) above,    wherein the functional layer comprises translucent inorganic    particles and/or translucent organic particles dispersed in a    transparent resin.-   (6) An optical sheet according to any one of (1) to (5) above,    wherein the functional layer is a transparent resin, and the    transparent resin is composed of a plurality of phase separable    resins.-   (7) An optical sheet according to (5) or (6) above, wherein the    refractive indexes of the transparent resin and the translucent    inorganic particles and/or translucent organic particles differ.-   (8) An optical sheet according to any one of (5) to (7) above,    wherein irregularities are produced in the surface of the functional    layer by the translucent inorganic particles and/or translucent    organic particles.-   (9) An optical sheet according to any one of (5) to (8) above,    wherein the difference in the refractive indexes of the transparent    resin and the translucent inorganic particles and/or translucent    organic particles is 0.01-0.25.-   (10) An optical sheet according to any one of (5) to (9) above,    wherein the mean particle size of the translucent inorganic    particles and/or translucent organic particles is 0.5-20 μm.-   (11) An optical sheet according to any one of (5) to (10) above,    wherein (d75-d25)/MV is no greater than 0.25, where MV is the mean    diameter based on the weight average of the translucent inorganic    particles and/or translucent organic particles, d25 is the    cumulative 25% diameter and d75 is the cumulative 75% diameter.-   (12) An optical sheet according to any one of (5) to (11) above,    wherein the translucent inorganic particles and/or translucent    organic particles are present at 1-30 wt % in the transparent resin.-   (13) An optical sheet according to any one of (1) to (12) above,    wherein the irregularities formed in a die surface are transferred    by inversion to form irregularities in the surface of the functional    layer.-   (14) An optical sheet according to any one of (5) to (11) above,    wherein the transparent resin is an ionizing radiation curable    resin, and the functional layer is formed by coating an ionizing    radiation curable resin composition containing the ionizing    radiation curable resin onto a transparent base material and    subjecting it to crosslinking curing.-   (15) An optical sheet according to (14) above, wherein the    transparent base material is composed of a cellulose-based resin,    the ionizing radiation curable resin composition comprises a solvent    that is to be impregnated into the transparent base material and/or    an ionizing radiation curable resin that is to be impregnated into    the transparent base material and a solvent that is not to be    impregnated into the transparent base material and/or an ionizing    radiation curable resin that is not to be impregnated into the    transparent base material, and the degree of impregnation into the    transparent base material is adjusted for control so that the    relationship of formula (I), formula (II) or formula (III) is    satisfied.-   (16) An optical sheet according to any one of (1) to (15) above,    wherein the transparent base material is triacetylcellulose or a    cyclic polyolefin.-   (17) An optical sheet according to any one of (1) to (15) above,    wherein the transparent base material is polyethylene terephthalate.-   (18) An optical sheet according to any one of (1) to (17) above,    wherein the functional layer comprises a hard coat layer, and the    steel wool scuff resistance is at least 200 g/cm².-   (19) An optical sheet according to any one of (1) to (18) above,    which has an anti-reflection functional layer formed on the    uppermost surface layer.-   (20) A polarizing plate employing an optical sheet according to any    one of (1) to (19) above.-   (21) An image display device employing a polarizing plate according    to (20) above.-   (22) A method for producing an optical sheet for use as a display    device surface, which has a functional layer on at least one side of    a transparent base material and has a diffusion factor on the outer    surface and/or interior of the functional layer, wherein the    production conditions are controlled so that the relationships of    formula (I) to formula (III) are satisfied.

According to the invention it is possible to provide an optical sheetwhich allows convenient evaluation of contrast that has not beenevaluable by the conventional haze value, and low reduction in contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between haze value andcontrast.

FIG. 2 is a graph showing the relationship between interior haze/totalhaze ratio and contrast.

FIG. 3 is a conceptual drawing showing the method of measuring diffusereflection intensity according to the invention.

FIG. 4 is a graph illustrating a method of calculating β.

FIG. 5 is a graph showing the correlation between S×β and contrast.

FIG. 6 is a graph showing the correlation between S×β and anti-glareproperty.

FIG. 7 is a graph showing the relationship between haze value andanti-glare property.

FIG. 8 is a graph showing the correlation between interior haze/totalhaze ratio and anti-glare property.

FIG. 9 is a set of diagrams illustrating the properties of reflectedlight based on the positional relationship of diffusion particles andsurface irregularities for projected light and external light.

FIG. 10 is a set of diagrams illustrating differences in diffusionproperties of light based on the difference in refractive indexes ofinterior diffusion particles and binder resin.

DETAILED DESCRIPTION OF THE INVENTION

The optical sheet of the invention has a functional layer on at leastone side of a base material and has a diffusion factor on the outersurface and/or interior of the functional layer, wherein therelationship 0.16<S×β<0.70 is satisfied.

The method of calculating S and β will now be explained with referenceto FIGS. 3 and 4.

(Method of Calculating S)

S is defined as the value of the diffuse regular reflection intensity ofthe optical sheet divided by the regular reflection intensity of thebase material which composes the optical sheet and has no diffusionfactor (diffuse regular reflection intensity/regular reflectionintensity of base material).

When visible light rays are irradiated from direction 4 onto the opticalsheet 1 as shown in FIG. 3, diffuse regular reflection occurs indirection 5, while part of the light is diffused. The direction of 5 isthe diffuse regular reflection direction, and the intensity of light inthe diffuse regular reflection direction is defined as the diffuseregular reflection intensity. As explained below, a visible lightray-absorbing material 8 such as a black acrylic board is attached tothe back side of the base material 2 via an adhesive, in order tosuppress back side reflection to match conditions of practical use.

In FIG. 3, the direction of 5 is the regular reflection direction whenmeasurement is conducted using a base material 2 having no diffusionfactor instead of the optical sheet 1, and the intensity of light in theregular reflection direction is defined as the regular reflectionintensity of the base material.

For a base material having irregularities on the surface, a transparentresin with a difference in refractive index of less than 0.02 from thebase material is coated onto the irregular surface and planarized, andthis is measured as the base material.

(Method of Calculating β)

When visible light rays are irradiated from direction 4 onto the opticalsheet 1 as shown in FIG. 3, diffuse regular reflection occurs indirection 5, while part of the light is diffused. According to theinvention, the reflection intensity R2 in the direction ±2° with respectto the diffuse regular reflection direction (where θ 2° in FIG. 3) andthe reflection intensity R1 in the direction ±1° with respect to thediffuse regular reflection direction (where θ is 1° in FIG. 3) are bothmeasured, and β is defined as the diffusion angle exhibiting ⅓ of thediffusion intensity extrapolated from a straight line connecting R2 andR1 to the diffuse regular reflection angle (hereinafter referred to as“virtual regular reflection diffusion intensity”) (see FIG. 4). Asexplained below, a visible light ray-absorbing material 8 such as ablack acrylic board is attached to the back side of the base material 2via an adhesive, in order to suppress back side reflection to matchconditions of practical use.

By controlling the material selection and production conditions in theoptical sheet production process, using S×β as the index, it is possibleto accomplish efficient production of an optical sheet that exhibits thefunction of a functional layer while also having low reduction incontrast.

Specifically, the diffuse reflection intensity is measured in thefollowing manner.

(Method of Measuring Diffuse Reflection Intensity)

The back side of an optical sheet (the side without the surface layer,or the side opposite the observer side) is attached via a transparentpressure-sensitive adhesive to a flat black acrylic board withoutirregularities or warping, to fabricate an evaluation sample. The blackacrylic board used here is to prevent back side reflection as describedabove, and it is not particularly restricted so long as no air layer ispresent on the back side of the optical sheet, and visible light can beabsorbed. When measurement is in the manufacturing line, for example, itis possible to perform online measurement by a method such as coating ablack paint onto the back side of the examining section of the opticalsheet.

Next, the evaluation sample is set in a measuring apparatus and a lightbeam is directed at an angle 45° from the normal to the surface of theoptical sheet side of the evaluation sample. The diffuse reflectionintensity is measured in a specific range with respect to light from alight beam impinging on the optical sheet surface of an evaluationsample and diffuse reflected, such as for example, in a range of −85° to85° with respect to the direction normal to the surface of the opticalsheet 1. Within this range, the reflection intensity in the direction±1° with respect to the direction of diffuse regular reflection and thereflection intensity in the direction ±2° with respect to the directionof diffuse regular reflection, are used to calculate the virtual regularreflection diffusion intensity, as explained above. In the case of lowdiffusion, the measurement range can be narrowed to an extent that doesnot significantly affect the results for the diffuse reflectionintensity, in order to shorten the time. The apparatus used to measurethe diffuse reflection intensity is not particularly restricted, but aGC5000L by Nippon Denshoku Industries Co., Ltd. was used for theinvention.

The reason that S×β determines the quality of contrast in an opticalsheet will now be explained. In an optical sheet, since the crests ofirregularities are dispersed and/or the diffusion particles are sparselydistributed in the interior in planar areas, the reflection diffusionproperty is a synthesis of external light that does not impact withdiffusion factors and is “through-light”, and light that impacts withdiffusion factors. Hence, the synthesis of sections having reflectionintensity only in the regular reflection direction and sections havingsmooth reflection intensity constitutes the reflectance property fororiginal regular reflection as shown in FIG. 4. On the other hand,contrast is determined by the diffusion property of light impacting thediffusion factors, and therefore the diffusion property for lightimpacting the diffusion factors must be known. That is, the reflectanceproperty for ideal regular reflection in FIG. 4 approximates the regularreflection of light impacting the diffusion factors.

Here, S×β represents the triangular area of the diagonally shadedsection in FIG. 4. Specifically, it approximates the area defined by thecurve representing the relationship of intensity with respect todiffusion angle, near the regular reflection of light impacting thediffusion factors. The reason for using the diffusion angle andintensity near regular reflection for the state of diffusion due todiffusion factors that affect contrast, is that the change in intensitydepending on diffusion angle due to the diffusion factors is highlyprominent near regular reflection. Thus, S×β represents the state of thediffusion angle and intensity due to diffusion factors near regularreflection, and it is therefore inferred to be equivalent to thediffusion performance of the diffusion factors. Furthermore, the factthat S×β has a suitable value, in terms of the surface form (externaldiffusion factor), is related to the irregularity slope angle and thepercentage of irregularities, while in terms of internal diffusion it isrelated to the difference in refractive indexes of the diffusionparticles and binder, the probability of collision with diffusionparticles and the shape, and in terms of interaction between the surfaceform and internal diffusion it is related to how much further theinteraction is weakened or strengthened, so that it determines thequality of contrast.

The optical sheet of the invention satisfies the following formula (I).0.16<S×β<0.70  (I)

When S×β is greater than 0.16, it is possible to obtain an optical sheetwith low reduction in contrast. From the viewpoint of limiting reductionin contrast, S×β is preferably greater than 0.26 (0.26<S×β), and morepreferably greater than 0.46 (0.46<S×β).

On the other hand, no reduction in contrast is seen with smaller S×βvalues, but because the optical sheet of the invention has a functionallayer and therefore has a diffusion factor, the lower limit variesdepending on the type of functional layer. In the optical sheet of theinvention it is essential for S×β to be less than 0.70. An S×β value ofless than 0.70 will result in a sufficient anti-glare property for thepurpose of use, in the case of an anti-glare sheet, for example.

In order to ensure that 0.16<S×β<0.70 according to the invention, thediffusion luminance distribution and intensity may be adjusted by theinternal diffusion factor and external diffusion factor.

The method for adjusting the reflective luminance distribution andintensity by the internal diffusion factor may be a method in whichtranslucent inorganic particles and/or translucent organic particles(hereunder also referred to simply as “translucent particles”) aredispersed in the resin composing the functional layer. It can also beaccomplished by controlling the form of the transparent resin composingthe functional layer and the translucent particles dispersed in thetransparent resin, the state of dispersion, the particle size, theamount of addition and the refractive index. The concentrations ofadditives other than the translucent particles added to the transparentresin can also influence the diffuse reflection intensity by theinternal diffusion factor.

As examples of methods for adjusting the diffuse reflection intensity bythe external diffusion factor there may be mentioned:

-   (1) a method of using a die with fine irregularities in the surface    and transferring the irregular shape to the optical sheet surface,-   (2) a method of forming irregularities in the surface by cure    shrinkage of the resin composing the functional layer, such as an    ionizing radiation curable resin,-   (3) a method of fixing the translucent fine particles as protrusions    from the surface layer to form irregularities in the surface (either    covering the protruding fine particles with the resin composing the    surface layer, or causing the fine particles to protrude out), and-   (4) a method of forming surface irregularities by external pressure.

As an example of method (1), an ionizing radiation curable resin may bemixed with the base material, and a die having fine irregularities maybe bonded to the coating layer of the ionizing radiation curable resinfor curing by ionizing radiation, to form an irregular shape on thesurface of the optical sheet.

Method (2) can yield fine irregularities with a smooth surface and istherefore effective for glare prevention, while method (3) allows theperformance to be adjusted by selection of the translucent particles andtransparent resin, the coating film thickness, the solvent, the dryingconditions and permeability into the base material, and is therefore ashorter process with simpler operation, which is thus effective forallowing low-cost production.

The functional layer provided between the irregular surface or irregularlayer and the transparent base material, or an anti-reflection layer,antifouling layer, hard coat layer, antistatic layer or the like, willalso influence the diffuse reflection intensity by the externaldiffusion factor. Specifically, by forming another functional layer onthe irregular surface to create a two-layer structure, it is possible tomoderate the surface irregularities and control the surface diffusion.Incidentally, by increasing the thickness of the coating film of theother functional layer, it is possible to moderate the surfaceirregularities, and control the surface diffusion by the coatingsolution composition and the coating and drying conditions as well.

Method (3) for obtaining the external diffusion factor is a suitablemethod from the viewpoint that it allows external diffusion and internaldiffusion to be imparted simultaneously by the type of translucent fineparticles used, thereby simplifying the production process.

On the other hand, using a method other than method (3) is preferredbecause it allows separate and independent design of a method foradjusting the diffuse reflection intensity by the external diffusionfactors and a method of adjusting the diffuse reflection intensity bythe internal diffusion factors, and this facilitates adjustment of theoptical performance other than contrast, such as resolution, glare andanti-glare properties. Furthermore, this allows adjustment of thediffuse reflection intensity by the external diffusion factor, withoutconsidering the optical performance of the resin that is used, thusfacilitating selection of a resin that exhibits physical performanceincluding surface resin hard coat property, antifouling property andantistatic property.

[Translucent Particles]

The preferred range for translucent particles dispersed in a transparentresin will now be explained in detail.

The translucent particles may be organic particles or inorganicparticles, and a mixture of organic particles and inorganic particlesmay also be used.

The mean particle size of the translucent particles used in the opticalsheet of the invention is in the range of preferably 0.5-20 μm and morepreferably 1-10 μm. Within this range it is possible to adjust thediffuse reflection intensity distribution by internal diffusion and/orexternal diffusion. In particular, if the mean particle size of thetranslucent particles is at least 0.5 μm the aggregation of particleswill not be excessive and it will be easy to adjust formation of theirregularities, while if it is no greater than 20 μm, images with glareand shine will be prevented and a greater degree of design freedom willbe ensured for the diffuse reflection intensity distribution.

Lower variation in the particle size of the translucent particles willalso result in lower variation in the scattering property, thusfacilitating design of the diffuse reflection intensity distribution.More specifically, (d75-d25)/MV is preferably no greater than 0.25 andmore preferably no greater than 0.20, where MV is the mean diameterbased on the weight average, d25 is the cumulative 25% diameter and d75is the cumulative 75% diameter. The cumulative 25% diameter is theparticle size constituting 25 wt %, counting from the particles withsmall particle size among the particle size distribution; and thecumulative 75% diameter is the particle size constituting 75 wt %,counting in the same manner.

As an example of adjusting the variation in particle size, the synthesisreaction conditions may be modified, while classification aftersynthesis reaction is also an effective means. With classification, thefrequency may be increased or the degree intensified to obtain particleswith the preferred distribution. The method used for classification ispreferably an air classification method, centrifugal classificationmethod, precipitating classification method, filtering classificationmethod, electrostatic classification method or the like.

The difference in refractive index between the transparent resincomposing the functional layer and the translucent particles ispreferably 0.01-0.25. If the difference in refractive index is at least0.01, it will be possible to prevent glare in the case of an anti-glareproperty sheet, for example, and if it is no greater than 0.25 thediffuse reflection intensity distribution design will be facilitated.From this viewpoint, the difference in refractive index is preferably0.01-0.2 and more preferably 0.02-0.15. The refractive index of thetranslucent particles is measured by measuring the turbidity withdispersion of equal amounts of the translucent particles in solventswith varying refractive indexes, obtained by varying the mixing ratiowith two different solvents having different refractive indexes,measuring the refractive index of the solvent at minimum turbidity usingan Abbe refractometer.

The diffuse reflection intensity can also be modified by using twodifferent types of translucent particles with a specific gravitydifference of 0.1 or greater, by using two different types oftranslucent particles with different particle sizes and a particle sizedifference of 0.5 μm or greater, by using two different types oftranslucent particles with a difference in refractive index of 0.01 orgreater, or by using spherical translucent particles and amorphoustranslucent particles together.

The specific gravity can be measured by liquid phase exchange or gasphase exchange (pycnometer method), the particle size can be measured bythe Coulter counter method or optical diffraction scattering method, orby observing the optical laminate cross-section with a microscope suchas an SEM or TEM, and the refractive index can be measured by directmeasurement with an Abbe refractometer, or by quantitative evaluation bymeasurement of the spectral reflection spectrum or spectroscopicellipsometry.

As translucent organic particles there may be used polymethylmethacrylate particles, polyacryl-styrene copolymer particles, melamineresin particles, polycarbonate particles, polystyrene particles,crosslinked polystyrene particles, polyvinyl chloride particles,benzoguanamine-melamine-formaldehyde particles, silicone particles,fluorine-based resin particles, a polyester-based resin, or the like.

As translucent inorganic particles there may be mentioned silicaparticles, alumina particles, zirconia particles, titania particles orhollow or porous inorganic particles.

Since even translucent fine particles having the same refractive indexand particle size distribution will have a different diffuse reflectionintensity distribution depending on the degree of aggregation of thetranslucent particles, the diffuse reflection intensity distribution canbe modified by combining two or more translucent particles withdifferent aggregation states, or using two or more types of inorganicparticles with different silane coupling treatment conditions to alterthe aggregation state.

In order to prevent aggregation of the translucent particles, it ispreferred to employ a method of adding silica with a particle size of nogreater than the wavelength of visible light rays, such as a particlesize of no greater than about 50 nm.

To obtain an internal diffusion effect, it is effective to use amorphoustranslucent particles of silica with a particle size of greater than thewavelength of visible light rays. Amorphous particles have an effect ofwidening the distribution of the transmission diffusion angle comparedto spherical particles. However, since amorphous translucent particlesalso widen the internal reflective distribution, they can affect thecoating film diffusibility and interfere with adjustment of the diffusereflection intensity, and therefore they are preferably added asnecessary, such as when a wide transmission diffusion is desired. Morespecifically, amorphous translucent particles are preferably added in arange of less than 4 wt % with respect to the total of the sphericalparticles and amorphous translucent particles.

The translucent particles are preferably added at 1-30 wt % and morepreferably 2-25 wt % in the transparent resin (solid content). At 1 wt %or greater it will be possible to produce sufficient anti-glareproperties or light diffusion, and to obtain satisfactory visibility,while reduction in contrast is avoided at up to 30 wt %.

[Transparent Resin]

The transparent resin used to form the functional layer may be anionizing radiation curable resin or thermosetting resin. For formationof the functional layer, a resin composition comprising the ionizingradiation curable resin or thermosetting resin may be coated onto a basematerial, and the monomer, oligomer and prepolymer in the resincomposition may be crosslinked and/or polymerized.

The functional groups of the monomer, oligomer and prepolymer arepreferably ionizing radiation-polymerizable, and are especiallyphotopolymerizable functional groups.

As photopolymerizable functional groups there may be mentionedunsaturated polymerizable functional groups such as (meth)acryloyl,vinyl, styryl and allyl.

As prepolymers and oligomers there may be mentioned acrylates such asurethane (meth)acrylates, polyester (meth)acrylates and epoxy(meth)acrylates, silicon resins such as siloxane, and unsaturatedpolyesters, epoxy resins and the like.

As monomers there may be mentioned styrene-based monomers such asstyrene and α-methylstyrene; acrylic monomers such as methyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, pentaerythritol(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritolpenta(meth)acrylate and trimethylolpropane tri(meth)acrylate; and polyolcompounds having two or more thiol groups in the molecule, such astrimethylolpropane trithioglycolate, trimethylolpropane trithiopropylateand pentaerythritoltetrathioglycol.

As binders there may be used polymers added to the resin composition.Polymethyl methacrylate (PMMA) is an example of such a polymer. Additionof a polymer allows the viscosity of the coating solution to beadjusted, and this is advantageous in that it can facilitate coatingwhile also facilitating modification of the irregular shape formed byparticle aggregation.

A photoradical polymerization initiator may also be added to the resincomposition if necessary. As photoradical polymerization initiatorsthere may be used acetophenones, benzoins, benzophenones, phosphineoxides, ketals, anthraquinones, thioxanthones, azo compounds and thelike.

As acetophenones there may be mentioned 2,2-dimethoxyacetophenone,2,2-diethoxyacetophenone, p-dimethylacetophenone, 1-hydroxy-dimethylphenyl ketone, 1-hydroxy-dimethyl-p-isopropyl phenyl ketone,1-hydroxycyclohexyl phenyl ketone,2-methyl-4-methylthio-2-morpholinopropiophenone,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone,4-phenoxydichloroacetophenone and 4-t-butyl-dichloroacetophenone, and asbenzoins there may be mentioned benzoin, benzoinmethyl ether,benzoinethyl ether, benzoinisopropyl ether, benzyldimethylketal,benzoinbenzenesulfonic acid ester, benzointoluenesulfonic acid ester,benzoinmethyl ether, benzoinethyl ether and the like. As benzophenonesthere may be used benzophenone, hydroxybenzophenone,4-benzoyl-4-2-methyldiphenyl sulfide, 2,4-dichlorobenzophenone,4,4-dichlorobenzophenone and p-chlorobenzophenone,4,4′-dimethylaminobenzophenone (Michler's ketone),3,3′,4,4′-tetra(t-butylperoxycarbonyl)benzophenone, and the like.

A photosensitizer may also be used therewith in combination, specificexamples of which include n-butylamine, triethylamine andpoly-n-butylphosphine.

Using a plurality of phase separable resins as the transparent resinwill also allow adjustment of the diffuse reflection intensity by theinternal diffusion factor. That is, by using a compatible component anda non-compatible component in admixture for the prepolymer, oligomer,monomer and polymer, it is possible to adjust the diffuse reflectionintensity by the internal diffusion factor. For example, when one resinis a styrene-based resin (polystyrene, styrene-acrylonitrile copolymeror the like), the other resin is preferably a cellulose derivative(cellulose ester such as cellulose acetate propionate or the like), a(meth)acrylic-based resin (polymethyl methacrylate or the like), analicyclic olefin-based resin (a polymer with norbornane as the monomer,or the like), a polycarbonate-based resin or a polyester-based resin.When one resin is a cellulose derivative (a cellulose ester such ascellulose acetate propionate or the like), the other resin is preferablya styrene-based resin (polystyrene, styrene-acrylonitrile copolymer orthe like), a (meth)acrylic-based resin (polymethyl methacrylate or thelike), an alicyclic olefin-based resin (a polymer with norbornane as themonomer, or the like), a polycarbonate-based resin or a polyester-basedresin.

The ratio of the combined resins (weight ratio) can be selected withinthe range of 1/99-99/1, preferably the range of 5/95-95/5, morepreferably the range of 10/90-90/10, even more preferably the range of20/80-80/20, and especially the range of 30/70-70/30.

In addition, using a prepolymer, oligomer or monomer with largepolymerization shrinkage will allow adjustment of the diffuse reflectionintensity by the external diffusion factor. A larger polymerizationshrinkage increases the surface irregularities, thus widening thediffuse reflection intensity distribution.

A solvent will usually be added to the radiation-curing resincomposition to adjust the viscosity or to allow dissolution ordispersion of each of the components. The type of solvent used willalter the surface condition of the coating film in the coating anddrying steps, and it is therefore selected as appropriate inconsideration of allowing adjustment of the reflection intensitydistribution by external diffusion. Specifically, it is selected inconsideration of the saturation vapor pressure and permeability into thebase material.

The resin composition used to form the functional layer composing theoptical sheet of the invention preferably contains an ionizingradiation-curable resin as the transparent resin, translucent particles,and a solvent. The resin composition preferably contains a solvent thatis to be impregnated into the base material (hereinafter also referredto as “permeable solvent”), and/or an ionizing radiation curable resinthat is to be impregnated into the base material, and a solvent that isnot to be impregnated into the base material and/or an ionizingradiation curable resin that is not to be impregnated into the basematerial. By adjusting the amount of impregnation into the base materialit is possible to control the thickness of the functional layer, andthus allow modification of the diffuse reflection intensity.

More particularly, the diffuse reflection intensity can be controlled bythe amount of impregnation into the base material and the sizes of thetranslucent particles. Specifically, when the amount of impregnation ofthe solvent and/or ionizing radiation curable resin (hereinafter alsoreferred to simply as “solvent mixture”) into the base material is lowand the translucent particle sizes are small, a functional layer isformed with the majority of the particles embedded in the solventmixture, but since the translucent particles tend to aggregate, thesurface irregularities are relatively large. On the other hand, whenusing a combination of a solvent mixture with a large amount ofimpregnation into the base material and translucent particles with smallparticle sizes, aggregation of the translucent particles is reduced andthe surface irregularities are relatively small.

When using a combination of a solvent and/or ionizing radiation curableresin with a large amount of impregnation into the base material andtranslucent particles with large particle sizes, the thickness of thefunctional layer is reduced, resulting in protrusion of the translucentparticles out from the functional layer, forming surface irregularitiesdue to the translucent particles. In contrast, when using a combinationof a solvent mixture with a small amount of impregnation into the basematerial and translucent particles with large particle sizes, thethickness of the functional layer is increased, thus inhibitingprotrusion of the translucent particles into the surface and resultingin relatively small surface irregularities.

By thus adjusting the amount of impregnation of the solvent and/orionizing radiation curable resin into the base material and effectingcontrol by combination with different particle sizes of translucentparticles, it is possible to form surface irregularity shapes of varioussizes.

This method is particularly effective when the base material is acellulose-based resin.

Furthermore, a single type of solvent may be used, or two or moredifferent solvents with different boiling points and/or relativeevaporation rates at ordinary temperature/ordinary pressure may beincluded. By using two or more different solvents, it is possible toachieve a wide range of control of the solvent drying speed. A highdrying speed results in volatilization, and thus less solvent and higherviscosity, before aggregation of the particles has occurred, such thatno further aggregation takes place. Thus, control of the drying speedaccomplishes control of the translucent particle sizes, and as explainedabove, it is linked with control of the diffuse reflection intensity bythe relationship with the degree of penetration of the solvent and/orionizing radiation curable resin into the base material. The relativeevaporation rate is the speed calculated by the following formulaaccording to ASTM-D3539, with a larger value representing fasterevaporation. Relative evaporation rate=time required for evaporation ofn-butyl acetate/time required for evaporation of solvent.

The specific solvent may be appropriately selected in consideration ofthe aforementioned explanation, and specifically there may be mentionedaromatic solvents such as toluene and xylene, and ketones such as methylethyl ketone (MEK), methyl isobutyl ketone (MIBK) and cyclohexanone. Anyof these may be used alone or in combinations of two or more. It ispreferred to use a mixture of at least one type of aromatic solvent andat least one type of ketone. To control the drying speed, there may becombined therewith a cellosolve such as methylcellosolve orethylcellosolve, a cellosolve acetate, or an alcohol such as ethanol,isopropanol, butanol or cyclohexanol.

Additives other than translucent particles may also be added to thetransparent resin in the optical sheet of the invention, as necessary.For example, various inorganic particles may be added to improve theoptical characteristics, including the physical properties such ashardness, and the reflectance and scattering property.

As inorganic particles there may be mentioned metals such as zirconium,titanium, aluminum, indium, zinc, tin and antimony, and metal oxidessuch as ZrO₂, TiO₂, Al₂O₃, In₂O₃, ZnO, SnO₂, Sb₂O₃, ITO, ATO or SiO₂.Also included are carbon, MgF, silicon, BaSO₄, CaCO₃, talc, kaolin andthe like.

The particle sizes of the inorganic particles are preferably asmicronized as possible in the resin composition during coating of thefunctional layer, in order to minimize the effect on the diffusereflection intensity distribution, and the mean particle size ispreferably in a range of no greater than 100 nm. By micronizing theinorganic particles to no greater than 100 nm, it is possible to form anoptical sheet that does not impair the transparency. The particle sizesof the inorganic particles can be measured by the light scatteringmethod or using an electron microscope photograph.

According to the invention, various surfactants may be used for ananti-aggregation effect and anti-settling effect, as well as to improvethe properties such as the leveling property. As surfactants there maybe mentioned silicone oils, fluorine-based surfactants, andfluorine-based surfactants preferably containing perfluoroalkyl groups.

According to the invention there may also be added stain-proofingagents, antistatic agents, coloring agents (pigments and dyes), flameretardants, ultraviolet absorbers, infrared absorbers, tackifiers,polymerization inhibitors, antioxidants, surface modifiers and the like.

The base material used in the optical sheet of the invention is notparticularly restricted so long as it is commonly used in opticalsheets, and it may be a transparent resin film, transparent resin plate,transparent resin sheet, transparent glass or the like.

As transparent resin films there may be used triacetylcellulose films(TAC films), diacetylcellulose films, acetylbutylcellulose films,acetylpropylcellulose films, cyclic polyolefin films, polyethyleneterephthalate films, polyethersulfone films, polyacrylic-based resinfilms, polyurethane-based resin films, polyester films, polycarbonatefilms, polysulfone films, polyether films, polymethylpentene films,polyetherketone films, (meth)acrylonitrile films, polynorbornane-basedresin films and the like. In particular, a TAC film or cyclic polyolefinfilm is preferred when the optical sheet of the invention is to be usedtogether with a polarizing plate, since these do not interfere withpolarized light, and a polyester film such as a polyethyleneterephthalate film is preferred if emphasis is on mechanical strengthand smoothness.

The base material may be a multilayer or monolayer material, and aprimer layer may also be provided on the surface for adhesion with thecoating film. In order to prevent interference patterns produced at theinterface when a substantial difference in refractive index existsbetween the base material and coating film layer, an anti-interferencepattern layer with a refractive index intermediate between the basematerial and coating film layer may be provided between them, orirregularities of about 0.3-1.5 μm may be formed as surface roughness(ten-point height of irregularities: Rz). The Rz value is Measuredaccording to JIS B0601 1994.

Functions such as a hard coat property, anti-glare property,anti-reflection property, antistatic property or antifouling propertymay be imparted to the optical sheet of the invention.

The hard coat property is usually evaluated based on the pencil hardness(measured according to JIS K5400), or by a 10-pass abrasion test usingsteel wool #0000 under a load, evaluating the maximum load under whichno damage is observed with black tape attached to the back side (steelwool scuff resistance). The pencil hardness of the optical sheet of theinvention is preferably H or greater, and more preferably 2H or greater.The steel wool scuff resistance is preferably 200 g/cm² or greater, morepreferably 500 g/cm² or greater and even more preferably 700 g/cm² orgreater.

For the anti-glare property, preferably translucent particles that canimpart an anti-glare property and a solvent are added to a transparentresin that can impart a hard coat property, and surface irregularitiesare formed by protrusions of the translucent particles themselves orprotrusions formed by aggregates of multiple particles. The anti-glareproperty can be evaluated by the following method.

(Method of Evaluating Anti-Glare Property)

A measuring sample is fabricated by attaching an optical sheet to a flatblack acrylic board. A light source is directed from a direction 15°with respect to the direction normal to the measuring sample, thereflected image from the light source is photographed with a CCD cameraset at a location in the mirror surface direction, and the maximum peakintensity of reflected light is determined. The measurement is conductedusing two light sources of different sizes, and the anti-glare propertyis evaluated by the following formula (IV), where PL is the reflectedlight peak intensity value as measured with the large light source, andPS is the reflected light peak intensity value as measured with thesmall light source.50×log (PL/PS)  (IV)A larger numerical value represents a higher anti-glare property. Thevalue for this evaluation is preferably 20 or greater, more preferably40 or greater and even more preferably 60 or greater.

Also, a measuring sample may be fabricated by attaching an optical sheetto a flat black acrylic board, a light source of about 900 lux projectedat an angle of about 15°, for example, and visual evaluation made of thedegree of halation seen from the regular reflection direction.

For an anti-reflection property, a low refractive index layer isprovided on the outer surface to reduce the reflectance of the sheet.The refractive index of the low refractive index layer is preferably nogreater than 1.5 and more preferably no greater than 1.45.

The low refractive index layer is formed of a material containing silicaor magnesium fluoride, or a fluorine resin as a low refractive indexresin.

The thickness d of the low refractive index layer preferably satisfiesd=mλ/4n. Here, m represents a positive odd number, n represents therefractive index of the low refractive index layer, and λ represents thewavelength. The value of m is preferably 1, and λ is preferably 480-580nm. From the viewpoint of low reflectance, the relationship 120<n·d<145is preferably satisfied.

Antistatic performance is preferably imparted from the viewpoint ofpreventing static electricity on the optical sheet surface. To impartantistatic performance, there may be mentioned methods known in theprior art, such as a method of coating a conductive coating solutioncomprising conductive fine particles, a conductive polymer, a quaternaryammonium salt, thiophene or the like and a reactive curing resin, or amethod of forming a conductive thin-film by vapor deposition orsputtering of a metal or metal oxide that forms a transparent film. Theantistatic layer may also be used as a portion of a functional layer,such as for hard coating, anti-glare, anti-reflection or the like.

The surface resistance value is an index of the antistatic property, andaccording to the invention the surface resistance value is preferably nogreater than 10¹²Ω/sq., more preferably no greater than 10¹¹Ω/sq. andespecially no greater than 10¹⁰Ω/sq. The “saturated electrostaticvoltage”, or the maximum voltage at which the optical film canaccumulate, is preferably no greater than 2 kV at an applied voltage of10 kV.

An antifouling layer may also be provided on the outer surface of theoptical sheet of the invention. An antifouling layer lowers the surfaceenergy and inhibits adhesion of hydrophilic or lipophilic contaminants.The antifouling layer can be imparted by adding a stain-proofing agent,and as stain-proofing agents there may be mentioned fluorine-basedcompounds, silicon-based compounds and their mixtures, among whichfluoroalkyl group-containing compounds are particularly preferred.

A method for producing an optical sheet of the invention will now beexplained in detail. According to the invention, it is essential tocontrol the production conditions so that the formula 0.16<S×β<0.70 asan index is satisfied, as mentioned above.

The optical sheet of the invention is produced by coating a resincomposition that is to form the functional layer on a base material. Thecoating method may be any of various known methods, such as dip coating,air knife coating, curtain coating, roll coating, wire bar coating,gravure coating, die coating, blade coating, microgravure coating, spraycoating or spin coating, for example.

According to the invention, the reflection diffusion luminance propertychanges by the coating amount, and therefore roll coating, gravurecoating or die coating is preferred since they allow a functional layerthickness to be stably obtained in the range of 1-20 μm.

After coating by any of the aforementioned methods, the sheet istransported into a heated zone to dry the solvent, or another knownmethod is used to dry the solvent. By selecting the relative evaporationrate of the solvent, the solid concentration, the coating solutiontemperature, the drying temperature, the drying air speed, the dryingtime and the dry zone solvent atmosphere concentration, it is possibleto adjust the external diffusion due to the profile of the surfaceirregularity shapes, and the internal diffusion due to the translucentparticles or additives. A method of adjusting the reflection diffusionluminance property by selection of the drying conditions is particularlypreferred and convenient. Specifically, the drying temperature ispreferably 30-120° C. and the drying wind speed 0.2-50 m/s, as thereflection diffusion luminance property can be controlled withappropriate adjustment in this range.

More specifically, increasing the drying temperature increases thepermeability of the resin and solvent into the base material. That is,by controlling the drying temperature it is possible to control thepermeability of the resin and solvent into the base material, and asexplained above, this is linked with control of the diffuse reflectionintensity by the relationship between the translucent particles andparticle sizes.

For example, when the resin composition used to form the functionallayer comprises a transparent resin, translucent particles and asolvent, the refractive index of the permeable component in thetransparent resin is lower than the refractive index of the translucentparticles and the leveling property and settling and aggregation of thetranslucent particles are on the same level, a longer drying time untilcuring results in permeation of the low refraction components in thetransparent resin into the base material, a higher refractive index ofthe transparent resin, and a lower difference in refractive index withthe translucent particles. On the other hand, since the proportion ofthe translucent particles with respect to the transparent resinincreases, the translucent particles tend to protrude out from thesurface, so that surface irregularities readily form. Thus, a longerdrying time reduces the internal diffusion while simultaneouslyincreasing the external diffusion. Incidentally, this permeability canbe utilized for adhesiveness between the base material and functionallayer by an anchor effect, or to prevent generation of interferencepatterns that become notable when the difference in refractive indexbetween the base material and functional layer is 0.03 or greater. Thispermeation layer that is produced by permeation of the low refractioncomponent in the transparent resin into the base material exhibits afunction as a refractive index-modifying layer wherein the refractiveindex between the base material and functional layer variescontinuously, and acts to eliminate the interface.

Also, by increasing the drying speed, the aggregation time of thetranslucent particles is shortened so that aggregation is impeded, thusexhibiting the same effect as an actual reduction in the particle sizeof the translucent particles. That is, by controlling the drying speedit is possible to control the sizes of the translucent particles thatare used, and as explained above, this is linked with control of thediffuse reflection intensity by the relationship with the degree ofpenetration of the solvent and/or ionizing radiation curable resin intothe base material.

EXAMPLES

The present invention will now be explained in greater detail byexamples, with the understanding that the invention'is in no way limitedby the examples.

(Evaluation Method)

1. Calculation of S

For the optical sheets fabricated in each of the production examples,the diffuse reflection intensity was measured using a GC5000L by NipponDenshoku Industries Co., Ltd., by the method described in the presentspecification, and S was calculated. The diffuse reflection intensitywas measured in a range of −85° to 85° with respect to the directionnormal to the surface of the optical sheet.

2. Calculation of β

For the optical sheets fabricated in each of the production examples,the diffuse reflection intensity was measured using a GC5000L by NipponDenshoku Industries Co., Ltd., by the method described in the presentspecification, and β was calculated. The diffuse reflection intensitywas measured in a range of −85° to 85°.

3. Measurement of Haze

For the optical sheets fabricated in each of the production examples,measurement was conducted with an HM-150 Hazemeter by Murakami ColorResearch Laboratory Co., Ltd.

4. Anti-Glare Property

For the optical sheets fabricated in each of the production examples, ameasuring sample was fabricated by attaching the optical sheet to a flatblack acrylic board, a light source of about 900 lux was projected at anangle of about 15°, for example, and the degree of halation seen fromthe regular reflection direction was visually evaluated. An evaluationof 1 represents the poorest anti-glare property, and an evaluation of 5represents a satisfactory anti-glare property.

5. Measurement of Contrast (Light-Room Contrast)

The light-room contrast is represented by the following formula.CR(L)=LW(L)/LB(L)(CR(L): Light-room contrast, LW(L): Light-room white luminance, LB(L):Light-room black luminance)

Since in general the change in light-room white luminance is small andthe change in light-room black luminance is large, the light-roomcontrast is governed by the light-room black luminance. Also, since theoriginal black luminance of the panel is smaller than the light roomblack luminance and can be ignored, the blackness (black luminance) inthe following regions was evaluated to evaluate the actual light-roomcontrast. For the optical sheets fabricated in each of the productionexamples, the back side (the side without the surface layer, or the sideopposite the observer side) is attached via a transparentpressure-sensitive adhesive to a flat black acrylic board withoutirregularities or warping, to fabricate an evaluation sample. The blackacrylic board used here is to prevent back side reflection as describedabove, and it is not particularly restricted so long as no air layer ispresent on the back side of the optical sheet, and visible light can beabsorbed. The sample was placed on the horizontal plane and subjected toorganoleptic evaluation (visual observation from an angle of about 45°with respect to the vertical axis on the side opposite the fluorescentlamp, at 50 cm above the sample surface) under 30 W three bandfluorescence (irradiation from a direction 45 degrees to the surface ofthe optical sheet), to evaluate the blackness on a 5-level scale. Anevaluation of 1 indicates poorest blackness and lowest contrast, and anevaluation of 5 indicates most satisfactory blackness and highestcontrast.

Production Example 1

Triacetylcellulose (80 μm thickness, FujiFilm Corp.) was prepared as abase material. The transparent resin used was a mixture ofpentaerythritol triacrylate

(PETA), dipentaerythritol hexaacrylate (DPHA) and polymethylmethacrylate (PMMA) (weight ratio: PETA/DPHA/PMMA=86/5/9) (refractiveindex: 1.51), and polystyrene particles (refractive index: 1.60, meanparticle size: 3.5 μm, (d75-d25)/MV=0.05) and styrene-acrylic copolymerparticles (refractive index: 1.56, mean particle size: 3.5 μm,(d75-d25)/MV=0.04) were added thereto as translucent particles, at 18.5and 3.5 parts by weight, respectively, with respect to 100 parts byweight of the transparent resin. A resin composition, obtained by mixinga mixed solvent of toluene (boiling point: 110° C., relative evaporationrate: 2.0) and cyclohexanone (boiling point: 156° C., relativeevaporation rate: 0.32) (weight ratio: 7:3) as the solvent at 190 partsby weight with respect to 100 parts by weight of the transparent resin,was coated onto the base material and dry air at 70° C. was circulatedat a flow rate of 0.2 m/s for 1 minute of drying. Next, it wasirradiated with ultraviolet rays (200 mJ/cm² under a nitrogenatmosphere) to cure the transparent resin, to fabricate an optical sheet(anti-glare sheet). The coating film thickness was 3.5 μm. The resultsof evaluating the optical sheet by the methods described above are shownin Table 2.

Production Examples 2-7 and Production Examples 9-17

An optical sheet (anti-glare sheet) was fabricated for ProductionExample 1, changing the type of base material, the type of transparentresin, the types and content of translucent particles, the type andcontent of the solvent, the drying conditions and the coating filmthickness, as listed in Table 1. The results of evaluating each opticalsheet in the same manner as Production Example 1 are shown in Table 2.

Production Example 8

Triacetylcellulose (80 μm thickness, FujiFilm Corp.) was prepared as abase material. Pentaerythritol triacrylate (PETA, refractive index:1.51) was used as the transparent resin, and there were added thereto astranslucent particles, styrene-acrylic copolymer particles (refractiveindex: 1.51, mean particle size: 9.0 μm, (d75-d25)/MV=0.04) andpolystyrene particles (refractive index: 1.60, mean particle size: 3.5μm, (d75-d25)/MV=0.05), at 10.0 parts by weight and 6.5 parts by weight,respectively, with respect to 100 parts by weight of the transparentresin. A resin composition obtained by mixing a mixed solvent of toluene(boiling point: 110° C., relative evaporation rate: 2.0) andcyclohexanone (boiling point: 156° C., relative evaporation rate: 0.32)(weight ratio: 7:3) as the solvent at 190 parts by weight with respectto 100 parts by weight of the transparent resin, was coated onto thebase material and dry air at 85° C. was circulated at a flow rate of 1m/s for 1 minute of drying. This was irradiated with ultraviolet rays(100 mJ/cm² under an air atmosphere) to cure the transparent resin (forman anti-glare layer).

A resin composition obtained by mixing PETA (pentaerythritoltriacrylate, refractive index: 1.51) as the transparent resin and amixed solvent of toluene (boiling point: 110° C., relative evaporationrate: 2.0) and cyclohexanone (boiling point: 156° C., relativeevaporation rate: 0.32) (weight ratio: 7:3) as the solvent at 190 partsby weight with respect to 100 parts by weight of the transparent resin,was coated onto the coating film layer (anti-glare layer), and dry airat 70° C. was circulated at a flow rate of 5 m/s for 1 minute of drying(hard coat layer formation). This was irradiated with ultraviolet rays(200 mJ/cm² under a nitrogen atmosphere) to cure the transparent resin,to fabricate an optical sheet (an anti-glare sheet with a hard coatlayer). The total coating film thickness was 13.0 μm. The results ofevaluating this optical sheet in the same manner as Production Example 1are shown in Table 2.

Production Example 9

An optical sheet (anti-glare sheet with a hard coat layer) wasfabricated for Production Example 8 in the same manner as ProductionExample 8, except that the content of the polystyrene particles as thetranslucent particles was 6.5 parts by weight with respect to 100 partsby weight of the transparent resin, and the total coating film thicknesswas 13.0 μm. The results of evaluation in the same manner as ProductionExample 1 are shown in Table 2.

TABLE 1 Transparent Drying conditions base Transparent Wind Film Prod.material resin Translucent particles Solvent Temp. Speed Time thicknessEx. Type Type Type Content Type Content (° C.) (m/s) (min) (μm)  1 TAC PA B 18.5 3.5 Y 190  70 0.2 1 3.5  2 TAC P A — 16 — Y 190  70 1 1 3.5  3TAC P B — 9 — Y 190  70 2 1 5.5  4 TAC Q C — 12 — Y 150  80 15 0.5 8.5 5 TAC Q E — 8 — X 190  70 10 0.5 2.0  6 TAC P A — 16 — Y 190  55 1 13.5  7 TAC P A B 16.5 2   Y 190  55 5 1 4.0  8 TAC Q/Q C A 10 6.5 Y/Y190/190 85/70 1/5 1/1 13.0  9 TAC Q E D 1 5   X 190  60 10 0.5 2.5 10TAC P A — 16 — Y 190  70 0.5 1 3.0 11 TAC P A B 12.5 2   Y 150 100 250.5 4.0 12 TAC Q E D 4 4   X 190  70 10 0.5 2.0 13 TAC Q E — 9 — X 190 70 10 0.5 2.0 14 TAC Q D E 3.5 0.5 X 150  80 20 0.5 2.5 15 TAC Q E — 2— X 150  80 20 0.5 4.0 16 PET Q D — 1 — X 190  70 5 1 4.5 17 PET Q E — 3— X 150  50 50 0.5 5.0 A: Polystyrene particles (refractive index: 1.60,mean particle size: 3.5 μm, (d75 − d25)/MV = 0.05) B: Styrene-acryliccopolymer particles (refractive index: 1.56, mean particle size: 3.5 μm,(d75 − d25)/MV = 0.04) C: Styrene-acrylic copolymer particles(refractive index: 1.51, mean particle size: 9.0 μm, (d75 − d25)/MV =0.04) D: Amorphous silica (refractive index: 1.45, mean particle size:1.5 μm, (d75 − d25)/MV = 0.6) E: Amorphous silica (refractive index:1.45, mean particle size: 2.5 μm, (d75 − d25)/MV = 0.8) P: Mixture ofpentaerythritol triacrylate (PETA), dipentaerythritol hexaacrylate(DPHA) and polymethyl methacrylate (PMMA) (weight ratio: PETA/DPHA/PMMA= 86/5/9) (refractive index: 1.51) Q: Pentaerythritol triacrylate (PETA)(refractive index: 1.51) X: Mixture of toluene (boiling point: 110° C.,relative evaporation rate: 2.0) and methyl isobutyl ketone (boilingpoint: 116° C., relative evaporation rate: 1.6) (weight ratio: 8:2) Y:Mixture of toluene (boiling point: 110° C., relative evaporation rate:2.0) and cyclohexanone (boiling point: 156° C., relative evaporationrate: 0.32) (weight ratio: 7:3)

TABLE 2 Interior Inte- haze/ Anti- Total rior Total Light- glare Prod.haze haze haze room prop- Ex. S β S × β (%) (%) (%) contrast erty  10.0540 2.910 0.157 40.6 29.2 71.9 1 5  2 0.0515 2.884 0.149 38.5 27.771.9 1 5  3 0.1806 1.954 0.353 10.5 7.9 75.2 3 2  4 0.0820 2.817 0.2312.5 1.6 64.0 2 4  5 0.0129 5.900 0.076 23.5 1.9 8.1 1 5  6 0.0283 4.6800.132 41.0 29.7 72.4 1 5  7 0.0291 4.572 0.133 44.6 32.7 73.3 1 5  80.4280 1.340 0.574 17.4 16.9 97.1 5 2  9 0.1821 1.641 0.299 2.2 1.3 59.13 3 10 0.0856 2.497 0.214 37.9 28.1 74.1 2 4 11 0.0481 2.813 0.135 42.034.4 81.9 1 5 12 0.0349 3.549 0.124 10.3 2.1 20.4 1 5 13 0.0125 6.1460.077 23.0 2.8 12.2 1 5 14 0.1074 1.946 0.209 3.6 2.8 77.8 2 4 15 0.07022.542 0.179 5.6 4.8 85.7 2 4 16 0.5940 1.183 0.703 1.3 0.0 0.0 5 1 170.1822 1.662 0.303 3.4 3.1 91.2 3 2

For Production Examples 1-17, S×β was calculated from the results ofmeasuring the diffuse reflection intensity, and the results of plottingS×β on the abscissa and contrast and anti-glare property on the ordinateare shown in FIG. 5 and FIG. 6, respectively. A correlation wasexhibited between S×β and contrast and anti-glare property. According tothe invention, Production Examples 3, 4, 8-10, 14, 15 and 17 correspondto examples that satisfy 0.16<S×β<0.70, and Production Examples 1, 2,5-7, 11-13 and 16 correspond to comparative examples that do not satisfythe formula. The light-room contrasts of the optical sheets of theexamples were satisfactory, while low values were exhibited forlight-room contrast by the optical sheets with S×β values of less than0.16. On the other hand, the optical sheets with S×β values of 0.70 orgreater had insufficient anti-glare properties.

FIG. 1 shows the results of plotting haze on the abscissa and contraston the ordinate, based on the results of measuring the haze values forProduction Examples 1-17. Also, FIG. 7 shows the results of plottinghaze on the abscissa and anti-glare property on the ordinate. Inaddition, FIG. 2 shows the results of plotting the ratio betweeninterior haze and total haze on the abscissa and contrast on theordinate, while FIG. 8 shows the results of plotting the ratio betweeninterior haze and total haze on the abscissa and anti-glare property onthe ordinate. No correlation was found between haze and contrast andanti-glare property, or between interior haze/total haze ratio andcontrast and anti-glare property.

The optical sheet of the invention has low reduction in contrast and asatisfactory anti-glare property.

The invention claimed is:
 1. An optical sheet having anti-glareproperties for use as a display device surface, the optical sheetcomprising: a base material; and a functional layer having an outersurface and an interior, the functional layer being formed by curing anionizing radiation curable resin on at least one side of the basematerial, and having a diffusion factor on at least one of the outersurface and the interior of the functional layer, wherein therelationship represented by the following formula (I) is satisfied,0.16<S×β<0.70  (I) S: Diffuse regular reflection intensity/regularreflection intensity of base material β: The diffusion angle exhibiting⅓ of the diffusion intensity, obtained by extrapolating a straight lineconnecting the reflection intensity for diffuse regular reflectionintensity at ±2 degrees and the reflection intensity for diffuse regularreflection at ±1 degree or the reflection intensity for diffuse regularreflection intensity at −2 degrees and the reflection intensity fordiffuse regular reflection at −1 degree, to the diffuse regularreflection angle.
 2. An optical sheet according to claim 1, wherein therelationship represented by the following formula (II) is satisfied;0.26<S×β<0.70  (II).
 3. An optical sheet according to claim 1, whereinthe relationship represented by the following formula (III) issatisfied;0.46<S×β<0.70  (III).
 4. An optical sheet according to claim 1, whereinthe display device is a liquid crystal display unit.
 5. An optical sheetaccording to claim 4, wherein the functional layer comprises at leastone of translucent inorganic particles and translucent organic particlesdispersed in the ionizing radiation curable resin, and irregularitiesdisposed on at least one of the outer surface and an inner surface ofthe functional layer by the at least one of the translucent inorganicparticles and the translucent organic particles.
 6. An optical sheetaccording to claim 4, wherein the base material is a transparent basematerial comprising a cellulose-based resin, the functional layer isformed by coating an ionizing radiation curable resin compositioncomprising the ionizing radiation curable resin on the transparent basematerial and subjecting the ionizing radiation curable resin tocrosslinking curing, the ionizing radiation curable resin compositioncomprises at least one of a solvent impregnated into the transparentbase material and an ionizing radiation curable resin impregnated intothe transparent base material, and at least one of a solvent notimpregnated into the transparent base material and an ionizing radiationcurable resin not impregnated into the transparent base material, andthe degree of impregnation into the transparent base material isadjusted for control so that the relationship of formula (I), formula(II) or formula (III) is satisfied.
 7. An optical sheet according toclaim 4, wherein the base material is a transparent base material thatis triacetylcellulose or a cyclic polyolefin.
 8. An optical sheetaccording to claim 4, wherein the base material is a transparent basematerial that is polyethylene terephthalate.
 9. An optical sheetaccording to claim 4, wherein the functional layer comprises a hard coatlayer, and the steel wool scuff resistance is at least 200 g/cm².
 10. Anoptical sheet according to claim 4, further comprising ananti-reflection functional layer formed on an uppermost surface layer.11. A polarizing plate employing an optical sheet according to claim 1.12. An image display device employing a polarizing plate according toclaim
 11. 13. An optical sheet according to claim 1, further comprisinga visible light ray-absorbing material attached to a back side of thebase material.
 14. An optical sheet according to claim 13, wherein thevisible light ray-absorbing material is attached to the back side of thebase material via an adhesive.
 15. An optical sheet according to claim13, wherein the visible light ray-absorbing material is a black acrylicboard.
 16. An optical sheet according to claim 1, wherein the at leastone of translucent inorganic particles and translucent organic particlesdispersed in the ionizing radiation curable are configured and arrangedso as to produce anti-glare properties.